Results

The first part of this presentation will provide an overview of the FAA’s part 23 rulemaking re-write which may serve as a model for re-write of other parts. The proposed icing regulation amendments and new industry standards will be covered. This will include supercooled large drop icing conditions and ice crystal conditions. A review of the service history of part 23 airplanes in icing and icing certification experience will be provided to justify the scope of the proposed amendments. Areas such as critical airframe ice accretions, propeller icing, autopilot operation, ice detection, ice protection system activation and operation, and low airspeed awareness are discussed. Since the new icing regulations will only be applicable to new airplanes and a limited number of modifications, the safety of the existing fleet in icing will also be holistically addressed. This discussion will include weather and pilot training.

In North America, about ten million kilograms of runway deicers are applied on airport runways to ensure safe takeoffs and landings of aircraft in adverse conditions. Although some of the chemicals are recovered, much of them are dispersed through aviation operations to airport’s surrounding environment. Little focus has been given into assessing and determining optimal quantities of deicers to be used on runways, that at the same time retain a high degree of safety, while reducing risks to the environment and improving airport efficiencies. Improved deicer performance tests would allow for the development of more environmentally sustainable deicers, through their improved performance. A better assessment of their deicing and anti-icing performance along with their degree of skid resistance on runway pavement, will help in the development of the next generation of runway de/anti-icing chemicals to ensure improved sustainable and safe aircraft takeoffs and landings.

Summary The size and shed time of ice shed from a propeller is predicted using a process that determines ice shape, ice growth rate and both internal and ice-structure interface stresses. A brittle failure damage model is used to predict the onset of local failure and to propagate damage in the ice until local ice shedding is obtained. Background Research into suitable ice-phobic coatings as a potential approach in an integrated aircraft ice protection system (IPS) has been ongoing for many years. Durability of these coatings has been an issue; however future research programmes such as the EU programmes AEROMUCO [1] and STORM [2] are looking to improve the Technology Readiness Level (TRL) of the application of these types of coatings. The introduction of ice-phobic coatings may make it possible to provide ice protection on rotating surfaces without the need for specialist ice protection systems.

A strong air/water interaction theory is used to develop a fast simplified model for the trapping of water in a film that flows over sub-grid surface roughness. The sub-grid model is used to compute correction factors that can alter mass transport within the film. This sub-grid model is integrated into a covariant film mass transport model for film flow past three-dimensional surfaces of a form suitable for aircraft icing codes. Sample calculations are presented to illustrate the application of the model. Aircraft icing codes usually consist of an aerodynamic solver, a droplet trajectory solver and a mechanism to grow the ice surface. Recently, icing codes have also made use of simple models for surface water transport, typically through a film lubrication model.

Certain operating modes of the Environmental Control System (ECS) of passenger aircraft are accompanied with significant frost formation in a number of pivotal parts of the system. These icing phenomena mostly occur during ground operation in hot regions with high humidity and are caused by the presence of remnant water in the preconditioned airstream. This water may appear – depending on the operating conditions – in the form of liquid water droplets, frozen particles and also water vapor. Icing conditions particularly prevail downstream of the AC packs and, as a consequence, ice formation takes place in the Pack Discharge Duct (PDD) and the mixing manifold. The ice thereby accumulates at the walls of the PDD and the mixing manifold and leads to a change of the flow geometry, which particularly increases the pressure drop in the system. This reduces the efficiency of the AC system and may even lead to a shutdown (of parts) of the system.

An important issue regarding landing performance is the reference speed which determines the approved fields lengths in which a landing can take place. The critical scenario is the accumulation of ice during the holding phase followed by descent, approach and landing. The effect of icing in the landing configuration, with the high-lift devices deployed, is relevant and should be anticipated during the early design phases by simulation. Due to the complex behaviour of the flowfield, 3D CFD methods has been used but that leads to a high computational cost which might be too intensive for the preliminary design phases . The purpose of this paper is to describe a lower cost procedure combining CFD and Quasi-3D modified Weissinger´s Method [3] which provides an accurate assessment of these effects to 5% margin in ∆CL , confirmed by wind tunnel testing.

While the industry is making consistent progress in predicting aerodynamic performance impact from ice accretion on rotor blade and ability to reliably design thermal anti-icing and/or deicing protection systems, ice shedding, natural or induced, is trailing behind both in terms of understanding the physics of impact ice adhesion and cohesion, mechanical fracture and energy dissipation upon impact on airframe or rotor systems. It is only recently that attention dedicated to the understanding of impact ice shedding on rotors has increased. Reference 1 summarizes the mechanical properties of ice. However, more recent test results (Reference 2 and 3) showed different results. It was therefore concluded that a data base more representative of helicopter operation was necessary. It is the intent of this paper to summarize the differences in test results and provide additional considerations for analytical modeling of the ice shedding process on a rotor blade.

The significant problem of engine power-loss and damage associated with ice crystal icing (ICI) was discussed in Mason et al [1]. These engine events included engine surge, stall, flameout, rollback and compressor damage and were connected to the ingestion of high concentrations of ice crystals associated with deep convective clouds. Since that time, several industry and government collaborations have taken steps to address the many technological requirements identified by the Engine Harmonization Working Group (EHWG) in 2007 [2]. The EHWG identified the need for in-situ measurements of ice concentration and size distribution to aid in the development of engine test facilities and methods to simulate the environment. Researchers are also addressing a second technology requirement identified by the EHWG: fundamental studies on the physics of ice accretion in the engine. Both efforts require study environments to be similar to the ones that cause in-service engine events.

Wind turbines mounted on cold climate sites are subject to icing which could significantly influence the performance of turbine blades for harvesting wind energy. To alleviate this problem, a number of techniques have been developed and tested. The currently used methods are surface coating, antifreeze chemicals, electrical resistance heating, hot air circulation, pulse electrothermal de-icing, manual chip-off, etc. Almost all thermal de-ice methods demand a high level of power to operate. Also, the high temperature induced to the blade by the thermal techniques may pose a risk for the integrity of composite blades. A relatively new strategy used for ice protection systems is ultrasonic guided waves (vibrations of very short length wave) on which a few research projects have been recently accomplished. This method is well known for non-destructive testing applications in which the waves typically propagate between 20 kHz and 100 kHz for long-range ultrasonic testing.

A new laser-scanning method was applied to capture the three-dimensional ice shapes created during ice-accretion test on a NACA 23012 airfoil conducted in the NASA Icing Research Tunnel. This facilitated the creation of high-fidelity digital surface representations of complex ice-shapes. Those digital shapes cannot only be used to create rapid-prototyping models for experimental simulations, but also serve as input for computational fluid dynamics (CFD) simulations. In the past, numerical simulations of iced airfoils and wings were typically based on simplified. But, independent of the complexity of the ice shape, current numerical methods often struggle with the complicated, highly unsteady separated flows occurring under iced conditions. In this work we present simulations based on the Lattice-Boltzmann methodology (LBM) and investigate it’s capability to simulate the flow around three-dimensional laser-scanned ice shapes.

Nomenclature β Local water catch efficiency h ice thickness (m) ρice Density of ice (kg/m3) LWC Liquid water concentration (kg/m3) V∞ freestream velocity (m/s) t time (s) Introduction Icing occurs when an aircraft flies through clouds containing supercooled water droplets with an ambient air temperature below the freezing point. Due to the inertia of water droplets they do not follow stream lines the result of this is that some water droplets hit the leading edge of the aircraft forming ice shapes. Those droplets that do not hit the leading edge get deflected away from the aircraft surface downstream. This will give region close to the aircraft surface where the amount of water droplets is depleted relative to the free stream values. This paper investigates the use of current icing codes used in industry to predict and model this shadow or depletion and overconcentration regions phenomena found close to a moving body in icing conditions.

This study aims to assess the effects of icing conditions on benchmark and real engine nacelle geometries. The calculations are done in both liquid and glaciated phase clouds. The computational tool used for prediction consists of four main modules; flow field solution, trajectory calculations, thermodynamic model and ice growth calculation. The flow solver in the current computational tool is a panel solver where the strengths of the singularity elements are varied in order to meet the required mass flow rate through the nacelle/engine. A RANS solver is in the process of being integrated into the current solver. The trajectory calculations are done both for liquid phase clouds including SLD effects like breakup and splash, and for solid/mixed phase clouds. The latter will take ice crystal drag coefficient, phase change, non-uniform flow field temperature, impact phenomena like erosion, bounce, etc. into account. The ice growth model is the Extended Messinger Model.

Icing phenomena have been studied since the middle of 1990s, and the numerical procedure for typical icing has been established. Recently, there are new problems of icing, which are SLD icing, ice crystal icing, and ice shedding phenomenon. The SLD and the ICI has been studied since 1990s. However, there are few researches on the ice shedding since the ice has many unknown physical parameters which are the density in atmosphere, the adhesion force between the wall and the accreted ice, the contact force between ice pieces and so on. Although existing icing models can simulate ice growth, these models do not have the capability to reproduce ice shedding. In the previous study, we developed an icing model that takes into account both ice growth and ice shedding. Furthermore, we validated the proposed ice shedding model through the comparison of numerical results and experimental data, which includes the flow rate loss due to ice growth and the flow rate recovery due to ice shedding.

The formation of ice over lifting surfaces can affect aerodynamic performance. In the case of helicopters, this loss in lift and the increase in sectional drag forces will have a dramatic effect on vehicle performance. The ability to predict ice accumulation and the resulting degradation in rotor performance is essential to determine the limitations of rotorcraft in icing encounters. The consequences of underestimating performance degradation can be serious and so it is important to produce accurate predictions, particularly for severe icing conditions. The simulation of rotorcraft ice accretion is a challenging multidisciplinary problem that until recently has lagged in development over its counterparts in the fixed wing community. But now, several approaches for the robust coupling of a computational fluid dynamics code, a rotorcraft structural dynamics code and an ice accretion code have been demonstrated.

Aircraft and engine manufacturers have to demonstrate satisfactory operation in a wide variety of environmental conditions, including flying through icing clouds. Use of icing simulation tools to assess the mass and shape of ice that builds on exposed surfaces helps in evaluating designs for operation in these conditions. The droplet collection efficiency is one of the parameters that help in calculating the amount of ice accretion on an exposed surface as it flies through icing clouds. An accurate estimate of the collection efficiency helps in the correct prediction of ice shape and mass which can be used for design evaluations. While high fidelity tools like CFD can be used to obtain a good estimate of the collection efficiency, these tools are very resource intensive and time consuming. A theoretical method or an empirical correlation helps in a very quick assessment of the collection efficiency, useful for quick calculations.

This paper will describe two recent modifications to the GlennICE software. First, a capability for modeling ice crystals and mixed phase icing has been modified based on recent experimental data. Modifications have been made to the ice particle bouncing and erosion model. This capability has been added as part of a larger effort to model ice crystal ingestion in aircraft engines. Comparisons have been made to ice crystal ice accretions performed in the NRC Research Altitude Test Facility (RATFac). Second, modifications were made to the runback model based on data and observations from thermal scaling tests performed in the NRC Altitude Icing Tunnel. Introduction Mason[1] describes a situation where an aircraft engine can encounter rollbacks and flameouts at high altitude conditions due to ice crystal ingestion. Numerous in-fight encounters had been observed. It was hypothesized that the cause of the incidents was the ingestion of a high volume of ice crystals into the engine.

As a result of a series of international collaborative projects to measure and assess aircraft icing environments that contain Supercooled Large Droplets (SLDs), it has been demonstrated that the current icing envelopes, e.g., Code of Federal Regulations (CFR) 14 Part 25 Appendix C, do not adequately capture conditions where SLDs are present. Consequently, regulatory authorities are considering extensions to the certification requirements to include SLD environments. In order to demonstrate compliance to an updated icing certification that includes SLD conditions, airframe and aircraft component manufactures will have an increased need for access to test facilities that can simulate this environment. To address this need, a series of tests have been conducted within the NRC’s Altitude Icing Wind Tunnel (AIWT) to examine the feasibility of expanding its current capabilities to include the SLD icing envelope.

Recent research on thermal ice protection of electrically heated restraining grids designed for applications in the environmental control system (ECS) of passenger aircraft is presented. The restraining grids consist of interlaced, electrically insulated wire (the topology of the grids is similar to that of tennis rackets) and are – in certain operation modes of the ECS – exposed to an airstream containing supercooled water droplets and/or ice particles. Heat is generated in the wire by an electric current, and the temperature of the wire is controlled with the aid of an electronic control system.

A numerical tool has been developed for predicting the unsteady behavior of the thermal wing ice protection systems (WIPS). The code was developed to account for a multi-layer composite structure. The performance predictions of a WIPS integrated into a metallic or into a composite structure can thus be achieved. The tool enables the simulation of unsteady anti-icing operations, for example, the WIPS may be activated with delay after entering into the icing conditions. In this case, ice starts to accrete on the leading edge before the WIPS heats up the skin. Another example is the ground activation of the WIPS for several seconds to check its functionality: low external cooling may cause high thermal constraints that must be estimated with accuracy to avoid adverse effects on the structure. The simulations give further opportunities compared to the current practice.

Ice Particle Impacts on a Flat Plate Mario Vargas, Peter M. Struk, Richard E. Kreeger, Charles Ruggeri, Mike Pereira, Duane Revilock National Aeronautics and Space Administration Glenn Research Center An experimental study was conducted at the Ballistic Laboratory of NASA Glenn Research Center to study the impact of ice particles on a stationary flat surface target set at 45 degrees with respect to the direction of motion of the impinging particle (Figure 1). The experiment is part of NASA efforts to study the physics involved in engine power-loss events due to ice-crystal ingestion and ice accretion formation inside engines. These events can occur when aircraft encounter high-altitude convective weather. The experiment was conducted to gain understanding of the physics involved when ice particles impact on a flat surface. Previous studies conducted by industry in the 1990s on the ingestion of ice particles in turbine engines were for hailstones.

Many studies have been performed to quantify the formation and evolution of roughness on ice shapes created in Appendix C icing conditions, which exhibits supercooled liquid droplets ranging from 1-50 um. For example Anderson and Shin (1997), Anderson et al. (1998), and Shin (1994) represent classical studies of ice roughness during short-duration icing events measured in the Icing Research Tunnel at the NASA Glenn Research Center. Because of the inability of the ice to survive the stylus profilometry measurement systems of the day and because of the lack of laser scanning systems, image analysis techniques were employed to characterize the roughness. Using multiple images of the roughness elements, the historical studies of roughness focused on extracting parametric representations of ice roughness elements.

The correct prediction of ice accretion on aircraft surfaces by simulation necessitates a good prediction of friction coefficient and heat transfer coefficient. After icing process, surface roughness induces high increase of friction and heat transfer, but simple Reynolds analogy is no longer valid. An experimental campaign is conducted in order to provide a database for numerical model development in the simple configuration of a flat plate under turbulent airflow conditions. The flat plate model is placed in the centre of the test section of a windtunnel with an improved temperature regulation. The test model is designed according to constraints for the identification of friction and heat transfer coefficients. It includes three identical resin plates which are moulded to obtain a specified roughness on the upper surface exposed to the flow. The latest resin plate is heated on its lower face by an electrical heater connected to a temperature regulator.

As ice begins to accrete on an aircraft in flight, the stochastic nature of the droplet impingement process dictates that the accreted ice is uneven along the surface resulting in roughness. Because of the varying convection along the surface and local shear rates along the surface, the resulting roughness statistical characteristics on an unswept wing are not constant along the streamwise direction. However, historical studies of roughness on iced airfoils performed in the NASA Icing Research Tunnel (IRT) at NASA Glenn Research Center employed image analysis approaches to create parametric representations of ice roughness element development over time. Because of the parametric descriptions and the limitations of the surface characterizations, ice roughness is often treated in analytical approaches and computational models as having constant parametric properties over the entire ice accretion area.

Icing is a phenomenon observed on aircraft airframes while flying through clouds of supercooled droplets. The phenomenon only occurs for ambient air temperatures below the freezing point. The droplets impinge on the aircraft surfaces and freeze, leading to ice accretion. The resulting change in aircraft geometry and surface roughness can modify the aircraft’s aerodynamic characteristics (lift loss, drag increase), it may affect air data probe measurements, and can even damage the engines by ice ingestion. In order to comply with certification regulations, airframers have to demonstrate safe operation of their aircraft in icing conditions. However, due to associated cost and time, it is prohibitive to cover the whole icing envelope by flight-testing or icing-tunnel testing. Therefore, aircraft manufacturers have developed, with support from research institutes, numerical prediction methods and tools to cover their prediction needs.

Icing is a serious problem that influences flight safety. It usually occurs when supercooled droplets in clouds are intercepted by aircraft wings, engine inlet, or other aircraft components. It not only causes weight/drag increase, but also reduces the lift, and leads to a degradation of the aircraft aerodynamic performance. Electrothermal heat source can keep the surface of aircraft components free of ice. It can also remove and shed moderate accreted ice on the surfaces. Continuous electrothermal heating can effectively protect the aircraft components from icing, however, this mode requires a great amount of energy, which would also increase the aircraft thermal load and reduce the economic efficiency. As a solution, periodic and partition electrothermal de-icing systems are developed. The main purpose of this paper is to investigate the ice melting process under periodic and partition electrothermal mode by numerical simulation based on the melting model.

Airframe icing is a topic of vital importance in aviation industry because it is mainly concerned with the safe and efficient operation of aircraft under all weather conditions. Over the last 15 years the role of supercooled large droplets (SLD) in aircraft icing has received increased attention. Recent meteorological investigations on icing weather have highlighted the existence of icing cloud characteristics beyond the actual certification envelope defined by the 14 CFR Part 25 Appendix C: Atmospheric Icing Conditions for Aircraft Certification, which accounts for an icing envelope characterized by water droplet diameters up to 50 μm. The mechanisms of impact and solidification of SLD are still not completely understood. The main subject of the present study is an investigation of impact of a supercooled drop onto a superhydrophobic substrate. Drop impact, spreading and rebound are observed using a high-speed video system.

This paper investigated impingement of supercooled large droplets onto smooth solid surfaces to understand the mechanism of splashing and secondary droplets formation using a novel moment of fluid (MOF) method. Previous studies have established a splashing threshold, but the effect of ambient gas in liquid droplet splashing is not fully understood. Our numerical results of water droplet splashing with relatively low velocity were consistent with experimental results: splashing occurs at high pressure but not at low pressure. Our simulation revealed that a thin film was formed after the droplet contacted the solid surface. The thin film moved at a lower speed at the contact with the solid due to viscous effect while the film moved at a higher speed away from the solid. As a result, air was trapped under the film, making the film floating on the air. When the pressure was high, the air density was high hence the aerodynamic forces by the air on the thin film.

There is significant recent evidence that ice crystals ingested by a jet engine at high altitude can partially melt and then accrete within the forward stages of the compressor, potentially producing a loss of performance, rollback, combustor flameout, compressor damage, etc. Several studies of this ice crystal icing (ICI) phenomenon have been conducted in the past 5 years using the RATFac (Research Altitude Test Facility) altitude chamber at the National Research Council of Canada (NRCC), which includes an icing wind tunnel capable at operating at Mach numbers (M), total pressures (po) and temperatures (To) pertinent to ICI. Humidity can also be controlled and ice particles are generated with a grinder. The ice particles are entrained in a jet of sub-freezing air blowing into the tunnel inlet. Warm air from the altitude cell also enters the tunnel, where it mixes with the cold ice-laden jet, increasing the wet-bulb temperature (Twb) and inducing particle melting.